Background: Lung cancer (LC) remains the leading cause of cancer-related mortality worldwide. Soluble receptor for advanced glycation end products (sRAGE) has emerged as a candidate biomarker in metabolic, inflammatory, and malignant diseases, although its prognostic significance in LC remains uncertain. Methods: Serum sRAGE levels were prospectively measured at baseline and prior to the second cycle of treatment in patients with non-small-cell lung cancer (NSCLC) and small-cell lung cancer (SCLC). Associations of sRAGE with overall survival (OS), progression-free survival (PFS), clinical features, and other biomarkers were analyzed. Results: In total, 42 patients were enrolled in this study. sRAGE was detected in 16 patients (38.1%) at baseline and in 15 patients (37.5%) after the first cycle of treatment. Pre-treatment sRAGE levels were strongly correlated with post-treatment levels (Pearson’s r = 0.78; 95% CI, 0.61–0.88; p = 4.1 × 10⁻⁹) and moderately correlated with PD-L1 tumor proportion score in NSCLC patients (Spearman’s ρ = 0.4, p = 0.049). Pre-treatment sRAGE levels tended to be higher in patients with indeterminate/high risk of liver fibrosis compared to patients with low risk (Wilcoxon rank-sum test, p = 0.041). Post-treatment change in sRAGE levels was correlated with whole blood cell count-derived inflammatory markers. A preliminary association between decreased sRAGE and overall survival in SCLC patients was observed. Conclusions: Serum sRAGE shows potential as a blood-based biomarker reflecting metabolic, immune, and inflammatory status in lung cancer, warranting further investigation to clarify its prognostic and therapeutic relevance.